I found the solution for your problem yesterday. Go to application, find cubase icon and right click on it. Select get info from the menu. Once a windows pops up select “low resolution”. I have been able to run huge projects at 32 samples and 48khz after doing this. And also no more CPU performance spikes.
At 96khz I could run at 64 samples. I was using a Motu audio express with firewire cable and thunderbolt adapter to connect to the mac.